System and method for detecting loss of input phase by sensing after power rectifier

ABSTRACT

A system for detecting a decrease in or loss of an input phase to a motor. A power rectifier rectifies and combines three input voltages to produce an output voltage to power the motor. A PFC circuit manages the power flowing to the motor. A sensing circuit located between the power rectifier and the PFC senses a voltage level of the power rectifier&#39;s output voltage. Alternatively, a sensing rectifier is connected before the power rectifier, and the sensing circuit senses the voltage level of the sensing rectifier&#39;s output voltage. A microprocessor compares the sensed voltage level to a threshold voltage level which is indicative of the decrease in or loss of one of the three input voltages, and if the former drops below the latter, then the microprocessor sends a signal to either shut off the motor or cause the PFC circuit to reduce the power flowing to the motor.

FIELD

The present invention relates to systems and methods for controlling theoperation of electric motors.

BACKGROUND

In some electric motor systems, alternating current (AC) powercomprising three input voltages, each 120 degrees out of phase with thenext, is converted by power rectifiers to direct current (DC) power fordriving electric motors. Under certain circumstances, such as anelectrical short, a phase can be lost. When a phase is lost, the motorcan still operate by drawing more current from the remaining phases, butthis can overheat the motor's components, especially when operating atfull load, and can even overload the transformer. In that light, it isdesirable to detect and address the loss of a phase. Prior attempts todetect the loss of a phase include examining the AC input voltagesbefore the power rectifier, with a drop in line voltage potentiallyindicating the loss of a phase, but this requires multiple isolated ordifferential circuits. It is also known to examine voltage peaks afterthe power rectifier, with the absence of an expected peak indicating theloss of a phase, but this requires using a timer to measure thefrequency of the voltage peaks.

Power Factor Correction (PFC) circuits are sometimes used to managepower flowing to motors. PFC makes a motor-driven appliance appear to bepurely resistive by eliminating any phase difference between the voltageand the current from the power supply, and thereby reduces energyconsumption by minimizing inefficient and costly reactive loads,maximizes the available power that can be drawn from the power supply,and minimizes any transient/harmonic effects that can feed back into theelectrical system and disrupt the power source to other appliances.Without PFC, an imbalance between input phases, resulting from adecrease in or loss of a phase, results in the current in thereduced/lost phase dropping very low or to zero such that the currentdrawn by the motor is pulled through the remaining phases. Some systemssense the bus voltage and when input voltage is low and load is highthey operate the PFC in a current-limited mode. However, running in thismode for an extended period of time may introduce undue stress oncomponents of the system and adversely affect the reliability andlongevity of the system. Importantly, this solution does not sense theloss of a phase in three-phase systems.

This background discussion is intended to provide information related tothe present invention which is not necessarily prior art.

SUMMARY

Embodiments of the present invention solve the above-described and otherproblems and limitations by providing improved detection of and responseto a decrease in or loss of an input voltage phase to a three-phasemotor so that overloading, overheating, and other harmful effects can beavoided. In one implementation, the present invention accomplishes thiswith a less complex and less expensive single sensing circuit that doesnot require isolation or differential sensing.

An electric motor system constructed in accordance with the presentinvention may broadly comprise an electric motor and a motor controlsubsystem. In a first embodiment, the motor control subsystem mayinclude a power rectifier operable to receive, rectify, and combine aplurality of spaced-apart sinusoidal input voltage signals (i.e., inputphases) to produce an output voltage signal to power the electric motor;a power factor correction circuit electrically connected to the powerrectifier so as to receive the output voltage signal and operable tomanage power flowing to the motor; a sensing circuit electricallyconnected between the power rectifier and the power factor correctioncircuit and operable to sense a voltage level of the output voltagesignal; and a microprocessor. The microprocessor may be in communicationwith the sensing circuit and the power factor correction circuit andoperable to receive the sensed voltage level from the sensing circuit,compare the sensed voltage level to a predetermined threshold voltagelevel, wherein the predetermined threshold voltage level corresponds toat least a decrease in one of the input phases, and if the sensedvoltage level is below the predetermined threshold voltage level, send asignal to the power factor correction circuit to at least limit anoutput power to the electric motor.

In a second embodiment, the motor control subsystem may include thepower rectifier operable to receive, rectify, and combine a plurality ofspaced-apart sinusoidal input voltage signals (i.e., input phases) toproduce a first output voltage signal to power the electric motor; thepower factor correction circuit electrically connected to the powerrectifier so as to receive the first output voltage signal and operableto manage power flowing to the motor; a sensing rectifier electricallyconnected in parallel with the power rectifier and operable to receive,rectify, and combine the plurality of spaced-apart sinusoidal inputvoltage signals to produce a second output voltage signal; and thesensing circuit electrically connected to the sensing rectifier andoperable to sense a voltage level of the second output voltage signal;and the microprocessor. The microprocessor may operate substantially thesame as in the first embodiment except that it examines the voltagelevel of the second output voltage signal of the sensing rectifierrather than the voltage level of the first output voltage signal of thepower rectifier.

In various implementations of these embodiments, the electric motorcontrol subsystem may further include any one or more of the followingadditional features. The electric motor may be a multiple—(e.g., three-)phase permanent magnet motor. The power factor correction circuit andthe sensing circuit may each use approximately the same referencevoltage. The predetermined threshold voltage level may correspond to aloss of one of the input phases. The predetermined voltage level may beadjustable. The signal sent by the microprocessor may cause the electricmotor to shut off. The process of comparing the sensed voltage level tothe predetermined threshold voltage level may be repeated apredetermined number of times during a predetermined period of time, andthe signal to at least limit the power flowing to the motor may only besent if the sensed voltage level is below the predetermined thresholdvoltage level for at least the predetermined number of times during thepredetermined period of time.

This summary is not intended to identify essential features of thepresent invention, and is not intended to be used to limit the scope ofthe claims. These and other aspects of the present invention aredescribed below in greater detail.

DRAWINGS

Embodiments of the present invention are described in detail below withreference to the attached drawing figures, wherein:

FIG. 1 is a schematic diagram of an electric motor system of the presentinvention;

FIG. 2 is a block diagram of a first embodiment of a motor controlsubsystem of the electric motor system of FIG. 1;

FIG. 3 is a schematic diagram of an implementation of a power rectifiercomponent of the motor control subsystem of FIG. 2;

FIG. 4 is a schematic diagram of an implementation of a sensing circuitof the motor control subsystem of FIG. 2;

FIG. 5 is a representation of a rectified three-phase output voltagesignal illustrating the loss of an input phase;

FIG. 6 is a flow diagram of process steps involved in the operation ofthe motor control subsystem of FIG. 2;

FIG. 7 is a flow diagram of steps performed in an implementation of theprocess of FIG. 6.

FIG. 8 is a block diagram of a second embodiment of the motor controlsubsystem of the electric motor system of FIG. 1; and

FIG. 9 is a schematic diagram of an implementation of a sensing powerrectifier circuit of the motor control subsystem of FIG. 8.

The figures are not intended to limit the present invention to thespecific embodiments they depict. The drawings are not necessarily toscale.

DETAILED DESCRIPTION

The following detailed description of embodiments of the inventionreferences the accompanying figures. The embodiments are intended todescribe aspects of the invention in sufficient detail to enable thosewith ordinary skill in the art to practice the invention. Otherembodiments may be utilized and changes may be made without departingfrom the scope of the claims. The following description is, therefore,not limiting. The scope of the present invention is defined only by theappended claims, along with the full scope of equivalents to which suchclaims are entitled.

In this description, references to “one embodiment”, “an embodiment”, or“embodiments” mean that the feature or features referred to are includedin at least one embodiment of the invention. Separate references to “oneembodiment”, “an embodiment”, or “embodiments” in this description donot necessarily refer to the same embodiment and are not mutuallyexclusive unless so stated. Specifically, a feature, structure, act,etc. described in one embodiment may also be included in otherembodiments, but is not necessarily included. Thus, particularimplementations of the present invention can include a variety ofcombinations and/or integrations of the embodiments described herein.

Broadly characterized, the present invention provides improved detectionof and response to a decrease in or loss of an input voltage phase to athree-phase motor so that overloading, overheating, and other harmfuleffects can be avoided. In one implementation, the present inventionaccomplishes this with a less complex and less expensive single sensingcircuit that does not require isolation or differential sensing.

Referring to the figures, an electric motor system 10 constructed inaccordance with the present invention is shown. Referring to FIG. 1, themotor system 10 may broadly include an electric motor 12; a power source14; and a motor control subsystem 16. Referring to FIG. 2, in a firstembodiment the motor control subsystem 16 may include a power rectifier20; a capacitor 22; a PFC circuit 24; a sensing circuit 26; and amicroprocessor 28. The motor 12 may be a three-phase permanent magnetmotor. For example, the motor 12 may be an approximately 0.5 to 2.5horsepower three-phase permanent magnet AC inverter motor. The motor 12may drive any appropriate load. For example, the motor 12 may drive afan or a pump which may be part of a heating and air-conditioning unit,such as a commercial blower, or an appliance, such as a washing machineor a clothes dryer, which may include additional electrical ormechanical components not described herein. The motor 12 may include ashaft 32 which transmits the driving force to the load. The power source14 may be a conventional AC power source, such as a standard 208 to 230Volt or 460 Volt source available in commercial buildings.

The power rectifier 20 may be operable to receive three-phase AC powerfrom the power source 14 and convert it to DC power for driving themotor 12. Referring to FIG. 3, one possible, non-limiting implementationof the power rectifier 20 of FIG. 2 is shown in greater detail. Otherimplementations are possible, and the details of any suchimplementations of the motor system 10 of the present invention willlargely depend on the requirements and functionalities of the system 10and its various components. The capacitor 22 may be operable to smooththe rectified three-phase voltage signal output by the power rectifier20. The PFC circuit 24 is located between the power rectifier 20 and thecapacitor 22, and may be operable to manage power flowing to the motor12 by reducing a phase difference between the voltage and the currentfrom the power supply 14 and thereby making the motor 12 appear to besubstantially electrically resistive.

The sensing circuit 26 is located between the power rectifier 20 and thePFC circuit 24, and may be operable to sense the voltage signal outputby the power rectifier 20 and provide input to the microprocessor 28regarding one or more a characteristics of the voltage signal. In oneimplementation, other control circuits (in addition to the PFC 24) maybe also located after (i.e., on the output side of) the power rectifier20, such that the sensing circuit 26, the PFC 24, and the other controlcircuits can use substantially the same reference voltage and thereforethe present invention does not require isolation or differentialsensing. Referring to FIG. 4, one possible, non-limiting implementationof the sensing circuit 26 of FIG. 2 is shown in greater detail. Otherimplementations are possible, and the details of any suchimplementations of the motor system 10 of the present invention willlargely depend on the requirements and functionalities of the system 10and its various components.

The microprocessor 28 may be operable to receive and process signalsfrom other components of the motor system 10, including the sensingcircuit 26; generate signals used to control operation of the motorsystem 10, including signals that control operation of the PFC 24; andexecute one or more computer programs, including control software,comprising executable instructions for accomplishing certain signalprocessing and generation and other functionality. In particular, themicroprocessor 28 executes a computer program that receives sensedvoltage data from the sensing circuit 26, and uses that data to detect adecrease in or loss of a phase and to respond thereto by generating acontrol signal that either shuts off the motor 12 or limits the inputcurrent and power on the remaining phases by causing the PFC 24 to limitoutput power to the motor 12.

In operation, the first embodiment may operate as follows. Referring toFIGS. 1, 2, 5, and 6, the power rectifier 20 receives three spaced-apartsinusoidal input voltage signals from the power source 14, rectifies andcombines them, and outputs an output voltage signal, as shown in step100, which, under normal operation, appears as the first portion of thewaveform shown in FIG. 5. If one of the three input voltage signals islost, the output changes to the second portion of the waveform shown inFIG. 5. The sensing circuit 26 senses the power rectifier's outputvoltage signal and communicates the output voltage level to themicroprocessor 28, as shown in step 102.

The computer program running on the microprocessor 28 compares thesensed output voltage level to a predetermined threshold voltage level,as shown in step 104. In one implementation, the predetermined thresholdvoltage level may be set relatively high in order to detect even a meredecrease in one of the three input voltage signals; in anotherimplementation, the predetermined threshold voltage level may be setrelatively low to detect a substantial or complete loss of one of theinput voltage signals. In FIG. 5, for example, the predeterminedthreshold voltage level may be set to a value that is between the troughvoltage (V1) associated with the normal output voltage signal and thetrough voltage (V2) associated with the output voltage signal when oneof the input voltage signals is missing. The predetermined thresholdvoltage level may be adjustable to accommodate, e.g., different motorsand operation conditions. Because the present invention examines thevoltage troughs rather than the voltage peaks, it is not frequencydependent so no timer is needed. If the sensed output voltage level isabove the predetermined threshold voltage level, then the system 10continues normal operation, as shown in step 106. However, if the sensedoutput voltage level drops below the predetermined threshold voltagelevel, the microprocessor 28 sends a signal either to shut off the motor12 or to cause the PFC 24 to limit input current and power on theremaining phases by limiting output power to the motor 12 (therebyallowing the motor 12 to continue operating at less than full power), asshown in step 108.

Referring to FIG. 7, one possible, non-limiting implementation of thefunctioning of the motor control subsystem 16 of FIG. 2 is shown ingreater detail. Other implementations are possible, and the details ofany such implementations of the motor system 10 of the present inventionwill largely depend on the requirements and functionalities of thesystem 10 and its various components. In operation, every approximately2 milliseconds the sensed output voltage level is compared to thepredetermined threshold voltage level, as shown in step 200. If thesensed output voltage level is below the predetermined threshold voltagelevel, then a counter signal is sent to increment a first counter, asshown in step 202. Simultaneously, a second counter counts from 0 to 500in approximately 1 second (i.e., the second counter is automaticallyincremented every approximately 2 milliseconds), then resets to 0 andrepeats, as shown in step 204. If the first counter does not reach apredetermined threshold number of detected low voltage levels before thesecond counter resets (i.e., within approximately 1 second), then thefirst counter also resets to 0 and the process repeats itself from thebeginning. However, if the first counter reaches a predeterminedthreshold number of detected low voltage levels before the secondcounter resets, as shown in step 206, then a signal is sent indicatingthat a low or missing phase has been detected, as shown in step 208.Thus, in this implementation, the sensed output voltage level must bebelow the predetermined threshold voltage level for a particular periodof time (which is controlled by the predetermined threshold number ofdetected low voltage levels) before the microprocessor 28 takes action,thereby ignoring single or short-term drops in the sensed output voltagelevel.

Any or all of these predetermined values, including the predeterminedthreshold voltage level, the frequency at which voltage levels arecompared, the reset time, and the predetermined threshold number ofdetected low voltage levels, may be stored in one or more memories, suchas electrically erasable read-only memories, that are accessible to themicroprocessor 28.

Referring to FIG. 8, in a second embodiment the motor control subsystem16 may include the power rectifier 20; the capacitor 22; the PFC circuit24; the sensing circuit 26; the microprocessor 28, and a sensingrectifier 30. The sensing rectifier 30 is electrically connected beforeand in parallel with the power rectifier 20 and may be similarlyoperable to receive three-phase AC power from the power source 14 andconvert it to DC power. Referring to FIG. 9, one possible, non-limitingimplementation of the sensing rectifier 30 of FIG. 8 is shown in greaterdetail. Other implementations are possible, and the details of any suchimplementations of the motor system 10 of the present invention willlargely depend on the requirements and functionalities of the system 10and its various components. In this second embodiment, the sensingcircuit 26 receives the output voltage signal of the sensing rectifier30, whereas in the first embodiment, the sensing circuit 26 receives theoutput voltage signal of the power rectifier 20. Thus, the sensedvoltage is based on the input to the power rectifier 20 rather than itsoutput. The second embodiment may be used when circuits and/or circuitcomponents, such as the large smoothing capacitor 22, following thepower rectifier 20 may interfere with sensing the troughs in the voltagesignal output by the power rectifier 20. The sensing circuit 26 maystill use substantially the same reference voltage as other controlcircuitry.

In various implementations of the second embodiment, the PFC circuit 24may be eliminated, the PFC circuit 24 may take the form of a boostconverter which converts lower voltage to higher voltage (thisimplementation may be used with, e.g., 230 Volt applications); and thePFC circuit 24 may take the form of a buck converter which convertshigher voltage to lower voltage (this implementation may be used with,e.g., 460 Volt applications).

In operation, the second embodiment may operate substantially as shownin FIGS. 6 and 7, but with the sensing rectifier 30 rather than thepower rectifier 20 performing step 100.

The present invention provides advantages over the prior art, includingthat it provides improved detection of and response to a decrease in orloss of an input voltage phase to a three-phase motor so thatoverloading, overheating, and other harmful effects can be avoided. Inone implementation, the present invention accomplishes this with a lesscomplex and less expensive single sensing circuit that does not requireisolation or differential sensing.

Although the invention has been described with reference to the one ormore embodiments illustrated in the figures, it is understood thatequivalents may be employed and substitutions made herein withoutdeparting from the scope of the invention as recited in the claims.

Having thus described one or more embodiments of the invention, what isclaimed as new and desired to be protected by Letters Patent includesthe following:
 1. An electric motor system comprising: an electricmotor; and a motor control subsystem including— a power rectifierconfigured to receive, rectify, and combine a plurality of spaced-apartsinusoidal input voltage phases to produce an output voltage signal topower the electric motor, a power factor correction circuit electricallyconnected to the power rectifier so as to receive the output voltagesignal, which has a pulsating waveform, and configured to manage powerflowing to the motor, a sensing circuit electrically connected at apoint between the power rectifier and the power factor correctioncircuit and configured to sense a voltage level of the output voltagesignal at the point, and a microprocessor in communication with thesensing circuit and the power factor correction circuit and configuredto receive the sensed voltage level from the sensing circuit, repeatedlycompare the sensed voltage level to a predetermined threshold voltagelevel which corresponds to a loss of one of the plurality ofspaced-apart sinusoidal input voltage phases, and if the sensed voltagelevel is repeatedly fluctuating above and below the predeterminedthreshold voltage level, send a signal to the power factor correctioncircuit to limit an output power to the electric motor.
 2. The electricmotor system as set forth in claim 1, wherein the electric motor is amultiple-phase permanent magnet motor.
 3. The electric motor system asset forth in claim 1, wherein the power factor correction circuit andthe sensing circuit each use a single reference voltage.
 4. The electricmotor system as set forth in claim 1, wherein the predeterminedthreshold voltage level is adjustable.
 5. The electric motor system asset forth in claim 1, wherein the signal sent by the microprocessorcauses the electric motor to shut off.
 6. A motor control systemconfigured to control an electric motor, the motor control systemcomprising: a power rectifier configured to receive, rectify, andcombine three spaced-apart sinusoidal input voltage phases to produce anoutput voltage signal to power the electric motor; a smoothingcapacitor; a power factor correction circuit electrically connectedbetween the power rectifier and the smoothing capacitor so as to receivethe output voltage signal, which has a pulsating waveform, andconfigured to manage power flowing to the motor; a sensing circuitelectrically connected at a point between the power rectifier and thepower factor correction circuit and configured to sense a voltage levelof the output voltage signal at the point; and a microprocessor incommunication with the sensing circuit and the power factor correctioncircuit and configured to receive the sensed voltage level from thesensing circuit, repeatedly compare the sensed voltage level to apredetermined threshold voltage level, which corresponds to a loss ofone of the three spaced-apart sinusoidal input voltage phases, and ifthe sensed voltage level is repeatedly fluctuating above and below thepredetermined threshold voltage level, send a signal to the power factorcorrection circuit to limit an output power to the electric motor. 7.The motor control system as set forth in claim 6, wherein the electricmotor is a three-phase permanent magnet motor.
 8. The motor controlsystem as set forth in claim 6, wherein the power factor correctioncircuit and the sensing circuit each use a single voltage.
 9. The motorcontrol system as set forth in claim 6, wherein the predeterminedthreshold voltage level is adjustable.
 10. The motor control system asset forth in claim 6, wherein the signal sent by the microprocessorcauses the electric motor to shut off.
 11. A motor control systemconfigured to control an electric motor, the motor control systemcomprising: a power rectifier configured to receive, rectify, andcombine three spaced-apart sinusoidal input voltage phases to produce aoutput voltage signal to power the electric motor; a smoothingcapacitor; a power factor correction circuit electrically connectedbetween the power rectifier and the smoothing capacitor so as to receivethe output voltage signal, which has a pulsating waveform, andconfigured to manage power flowing to the motor; a sensing circuitelectrically connected at a point between the power rectifier and thepower factor correction circuit and configured to sense a voltage levelof the output voltage signal at the point; and a microprocessor incommunication with the sensing circuit and the power factor correctioncircuit and configured to receive the sensed voltage level from thesensing circuit, repeatedly compare the sensed voltage level to apredetermined threshold voltage level, which corresponds to a loss ofone of the three spaced-apart sinusoidal input voltage phases, and ifthe sensed voltage level is repeatedly fluctuating above and below thepredetermined threshold voltage level, send a signal to shut off theelectric motor.
 12. The motor control system as set forth in claim 11,wherein the electric motor is a three-phase permanent magnet motor. 13.The motor control system as set forth in claim 11, wherein the powerfactor correction circuit and the sensing circuit each use a singlereference voltage.
 14. The motor control system as set forth in claim11, wherein the predetermined threshold voltage level is adjustable.